This invention relates generally to a pneumatic tire, and more particularly to a pneumatic tire for a passenger car which reduces ply steer of the tire to improve straight running stability and drastically, driving comfortability.
A radial tire for a passenger car in accordance with the prior art generally has a construction in which a belt reinforcing layer consisting of at least two layers is interposed between a tread and a carcass cord layer. Reinforcing cords of one of these belt reinforcing layers have a cord angle of from 15.degree. to 30.degree. with respect to the circumferential direction of the tire while the reinforcing cords of the other have a cord angle of from 150.degree. to 165.degree. with respect to the tire circumferential direction, and these reinforcing cords cross one another. The carcass cord layer consists of one or two layers, and the cords of each layer are disposed at angle of about 90.degree. with respect to the tire circumferential direction. In comparison with a bias tire, the radial tire of this kind is more excellent in the aspects of brake performance, low fuel consumption and wear resistance due to the effect brought forth by the belt reinforcing layers, but has a problem in that straight running stability is lower due to the belt reinforcing layers. In other words, when the radial tire rotates and advances, a lateral force occurs either to the right or left to the advancing direction, even if a slip angle is zero, so that a car advances in a direction different from the direction in which the driver of the car wishes to drive the car.
Generally, the lateral force with the zero slip angle consists of force components resulting from two different mechanisms. One is called "conicity" (CT) with the other being called "ply steer" (PS), and they are classified as part of the uniformity characteristics of the tire. On the other hand, in accordance with a uniformity test method (JASO C607) for a car tire, the conicity CT and the ply steer PS are expressed by the following equation from their definition where LFD represents the mean value of the lateral force when the tire rotates once, LFDw represents the value when measured on the face and LFDs represents the value when measured on the reverse by changing the position of the tire: EQU LFDw=PS+CT (1) EQU LFDs=PS-CT (2)
PS and CT can be obtained as follows from equation (1) and (2): ##EQU1##
Each relation between (1) through (4) is illustrated in FIG. 1.
Among the conicity and the ply steer described above, the conicity is believed to be a force that is generated because the tire shape is geometrically asymmetric with respect to the center of the circumferential direction of the tire, that is, a force that is generated when the tire assuming the shape of a circular truncated cone rolls. The main reason for the occurrence of this force is the position of the belt reinforcing layer inserted into the tread of the tire, and hence can be reduced by somehow improving the production procedures. In contrast, the ply steer is a force that is inherent to the structure of the belt reinforcing layer, and can not be drastically reduced in practice unless the structure itself of this belt reinforcing layer is changed.
Let's consider the belt reinforcing layer. The belt reinforcing layer can be expressed as a two-layered laminate sheet 50 consisting of belt reinforcing layers 50u and 50d, as depicted in FIG. 2(A). It is well known in the art that when a tensile force is caused to act upon this two-layered laminate sheet 50 in the tire circumferential direction EE', the two-layered laminate sheet 50 undergoes not only deformation inside the two-dimensional plane on which the tensile force acts but also twist deformation three-dimensionally of the plane as depictied in FIG. 2(B). The ply steer described above results from this twist deformation of the belt reinforcing layer.
Various proposals have been made in the past so as to reduce the ply steer by adding a new belt reinforcing layer to the existing belt reinforcing layer, but the addition of the new belt reinforcing layer is not much desirable because it deteriorates the characterizing features of the radial tire such as its low fuel consumption, and the like.
The inventors of the present invention made intensive studies in order to eliminate the problem described above, and as a result, proposed previously a pneumatic tire (Japanese Patent Laid-Open No. 114704/1982). In the pneumatic tire of this prior patent application which is equipped with the belt reinforcing layer of the structure described above and with a carcass cord layer consisting of upper and lower two layers, the angle of reinforcing cords constituting each carcass cord layer with respect to the tire circumferential direction, when measured from the side in which the reinforcing cords of the belt reinforcing layer on the contact side with the carcass cord layer describe an acute angle with respect to the tire circumferential direction, is such that the mean value 1/2(.alpha..sub.1 +.alpha..sub.2) of the angle .alpha..sub.2 of the reinforcing cords of the carcass cord on the contact side with the belt reinforcing layer and the angle .alpha..sub.1 of the reinforcing cords of the carcass cord below the former is from 96.degree. to 108.degree. and their difference (.alpha..sub.2 -.alpha..sub.1 ) is from 10.degree. to 40.degree.. According to this arrangement, the ply steer can be reduced without adding afresh any belt reinforcing layer, and straight running stability due to the belt reinforcing layer can be improved drastically.
However, when the reinforcing cords of each carcass cord layer are arranged to cross one another at a predetermined angle in the tire circumferential direction in order to improve the straight running stability as described above, another problem develops in that the reinforcing cords of each carcass cord layer are cut due to compression, particularly when driving at a low internal pressure of the tire.